1. Field of the Invention
This invention relates to the gasification of black liquor. In one of its more particular aspects, this invention relates to a process in which aqueous black liquor is gasified by means of a molten salt.
2. Prior Art
In the production of pulp and paper using the sodium based sulfate and sulfite processes, digestion of wood with aqueous alkaline solutions results in the production of a byproduct which is known as spent or black liquor, hereinafter referred to as black liquor. This byproduct is considered a waste material and must be converted into useful products in order to realize economies in the overall pulping process. In particular, it is desired to regenerate sodium sulfide, which can be used to reconstitute active solutions for the pulp digestion step of the process. In addition, it is desirable to utilize black liquor as an energy source.
The most widely practiced method of processing black liquor makes use of the Tomlinson recovery furnace. In this process concentrated black liquor is burned in the furnace of a specially designed boiler to produce steam; a molten salt product called "smelt", which contains sodium carbonate and sodium sulfide; and non-combustible flue gas which, after suitable cleaning, is vented to the atmosphere. The process has served the pulp and paper industry for about fifty years, yet it has serious deficiencies. The large volume of flue gas is difficult to clean and can constitute an environmental problem; all recovered energy is in the form of steam which has limited utility; explosions can occur if the boiler tubes leak and cause water to contact the smelt; and the reduction of sulfur compounds to sulfide is incomplete.
Various other processes and improvements to the Tomlinson furnace have been used or proposed for converting black liquor to useful products.
U.S. Pat. No. 1,808,773 discloses a process which utilizes a black liquor recovery furnace having two zones of combustion. In the first high temperature combustion zone, black liquor sprayed into the furnace is dehydrated and substantially completely burned. In the second zone an additional quantity of black liquor is sprayed into the furnace along with sodium sulfate. In this zone water is removed from the black liquor by evaporation and partial combustion of the black liquor results in the formation in the bottom of the furnace of a smelting bed of spongy carbon, mixed with alkali residues from black liquor and added sodium sulfate. Reducing conditions maintained in the bottom of the furnace result in the reduction of sulfate to sulfide. Although this process results in the conversion of sodium sulfate to sodium sulfide and the combustion of black liquor, the percentage of unconverted sulfate is relatively high, ranging from 8-12%.
U.S. Pat. No. 2,056,266 describes the use of a combined smelter and boiler furnace for recovering alkali metal values from black liquor and utilizing the heat content thereof. In a fuel bed zone black liquor solids are burned in a reducing atmosphere with the result that partially burned gases rise from the fuel bed and are completely burned by introducing a stream of air into a combustion zone above the bed. The combustion zone contains boiler tubes for the production of steam. Flue gases produced in the combustion zone are allowed to rise and an inert gas is blown down on the fuel bed to prevent entrainment of solids in the gases rising from the fuel bed and to create a distinct line of separation between zones. Fused alkaline values are drained from the bottom of the bed. Although this process provides means of recovering alkali metal values from black liquor and utilizing at least some of the heat content thereof, the apparatus necessary for carrying out the process is complex and requires a separate means of drying black liquor.
U.S. Pat. No. 2,182,428 discloses a process for drying waste liquors by spraying the liquor to be evaporated upon the surface of a heat transfer medium such as an oil, tar, pitch, asphalt or wax. Since the heat transfer medium is inert, the waste liquors are merely evaporated without recovering any other useful product.
None of the processes-previously available are capable of conveniently recovering substantially the entire energy and chemical content of black liquor as high value products.
U.S. Pat. No. 3,916,617, assigned to the same assignee as the present invention, describes the use of a molten salt to produce a low Btu gas from the gasification and partial oxidation of a carbonaceous material.
In U.S. patent application Ser. No. 350,560, assigned to the same assignee as the present invention, the gasification of dried black liquor solids in a molten salt pool is described. In this process, a combustible offgas is produced and a high level of reduction of the sulfur content of the black liquor solids to sulfide is realized. The drying of black liquor to form the black liquor solids which are required as feed to the process of this invention, however, involves complex process steps in addition to the ordinary use of multiple effect evaporators which produce a concentrated black liquor having a solids content of 45-75%.
It would be desirable to provide a process which is operable upon readily available concentrated black liquor streams, which recovers in the form of a readily useable fuel the heat content of black liquor to the maximum extent and which results in a very high percentage of reduction of sulfur containing compounds present in the black liquor to sulfide.